The present invention is generally related to clarifying agents or transparency enhancing agents used in conjunction with polyolefin resins, such as polypropylene. More specifically, the present invention includes an additive that is used in conjunction with certain dibenzylidene sorbitol-type (DBS) clarifiers within polyolefin resins to reduce the release of aldehydes formed as a result of the use of such clarifying agents.
Various techniques have been used in the past to clarify different types of plastics, such as polypropylene. One commercially successful method for clarifying plastics is by using a form of dibenzylidene sorbitol as a clarifying agent. One such clarifying agent, bis(3,4-dimethylbenzylidene) sorbitol acetal (alternatively referred to as 3,4-DMDBS), and its methods of use are described in detail in U.S. Pat. No. 5,135,975 and 5,049,605, both issued to Rekers. As used herein, the term polyol actetal refers to the reaction product of a polyol and an aldehyde, which includes dibenzylidene sorbitol (DBS) compounds and dibenzylidene xylitol (DBX) compounds. All patents mentioned herein are incorporated by reference in their entirety.
One problem that is generally associated with the use of other types of DBS clarifying agents, such as 4-methyl DBS (alternatively referred to as MDBS), is that certain by-products are produced, such as aldehydes, which tend to migrate out of the plastic after formation thereof. 4-Ethyl DBS, 4-chloro DBS, and 2,4-dimethyl DBS are other clarifiers with similar deficiencies. Certain sorbitol acetal clarifiers, including 4-methyl DBS, are not completely heat or acid stable and partially decompose somewhat during the processing of polyolefins, which produces the undesirable by-product of aldehydes.
This release of aldehydes is usually noticeable to users as a smell or taste, and may significantly impact the smell and taste of foods stored in a plastic container made using certain DBS clarifying agents.
It would be desirable to prevent the release of aldehydes from clarified plastics, particularly in applications where the plastic is used for food containers, and other uses where food will come into contact directly with the clarified plastic. Other applications where it is absolutely crucial to prevent release of aldehydes from plastics includes the medical field, for such uses as pre-filled syringes, etc. Several attempts have been made to address this problem, with varying degrees of success.
Different types of DBS agents have been used as plastic clarifiers, including alkyl substituted DBS compounds, which may decompose during melt processing, resulting in the formation of substituted benzaldehydes. One approach for addressing this problem has been to try to stabilize the DBS agents, for instance, by co-compounding them with certain acid acceptors, as disclosed in U.S. Pat. No. 4,419,473, issued to Mahaffey, Jr.
Other attempts to stabilize DBS agents are disclosed in Japanese patent applications Hei 9[1997]-286787 and Hei 9[1997]-286788, by Ikeda et al. These patent applications are directed to stabilizing DBS agents by adding alkali salts of amino acids, and alternatively adding polyols to the polypropylene formulations, respectively. Matsuhira discloses pre-treatment of neat DBS powder with hydroxylamine or hydrazine derivates to reduce aldehyde content before the clarifier is compounded, in Japanese patent applications Sho 60[1985]-32791 and Sho 60[1985]-42385.
U.S. Pat. No. 5,001,176, issued to Nakazima, is directed to a crystalline polyolefin composition that contains a dibenzylidene sorbitol type compound and a cyclodextrin. U.S. Pat. No. 5,856,385, issued to Mehrer, relates to the use of nucleating agents based on dibenzylidene sorbitol, or derivates thereof, in combination with amido-functional compounds.
None of the prior art, however, discloses the use of hydrazide or hydrazine agents as an additive to a DBS enhanced polyolefin resin in order to prevent eventual release of malodorous and foul tasting aldehyde compounds from the final product. As used herein, the terms hydrazine and hydrazide refer to compounds that contain singly bonded nitrogens, one of which is a primary amine functional group.
Accordingly, it is an important object of the present invention to provide an additive to be used with dibenzylidene sorbitol-type clarifying agents for polyolefin resins for the purpose of reducing the amount of aldehydes released from the final plastic product. Another important object of the present invention is to provide a compound that reacts with aldehydes to form a water insoluble product, which is chemically stable within DBS clarified plastic products. Yet another important object of the present invention is to provide an additive to be used in conjunction with DBS clarifying agents to prevent undesirable smells and tastes from being released from a final plastic product. Still another important object of the present invention is to provide an aldehyde scavenging compound that will not significantly affect the transparent optical properties of a clarified polymer, such as polypropylene. Another important object of the present invention is to provide an additive that is effective in reducing or eliminating the release of aldehydes from plastics treated with a variety of different DBS-type clarifying agents. Yet another important object of the present invention is to provide a mixture and method for clarifying polyolefin products, which provides a significantly reduced level of released aldehydes than prior attempts have yielded.
These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims.
In order to provide an additive for DBS-type clarifiers to reduce the malodorous and foul tasting aldehydes from migrating out of plastics, it is necessary that such an additive exhibit certain qualities. It must react selectively with aldehydes to form a water insoluble, non-toxic product that is stable within a polypropylene polymer matrix. Further, such an additive must not negatively affect the optical transparent properties imparted to the polypropylene by the DBS-type clarifying agent. The desired additive should be inexpensive to manufacture in commercial quantities, as well.
It has been found that the addition of certain hydrazide compounds to DBS enhanced polypropylene products in combination with certain acid scavengers significantly reduces the eventual release of aldehydes from the plastic after formation thereof. Suitable acid scavengers useful for this purpose include, without limitation, hydrotalcite, sodium stearate, calcium stearate, lithium stearate, and calcium stearyl lactate. As used herein, an aldehyde scavenger or acid scavenger is defined as a compound that readily reacts with aldehydes or acids, respectively. Also, the terms xe2x80x9chydrazidexe2x80x9d or xe2x80x9chydrazide compoundxe2x80x9d are defined herein as encompassing dihydrazide compounds as well.
Specifically, it is believed that certain hydrazide compounds form a water insoluble reaction product with aldehydes within polypropylene products, thereby eliminating or significantly reducing the levels of aldehydes released from the plastic. Both hydrazides and hydrazines are known to react readily with aldehydes to form hydrazones.
A group of hydrazide compounds was synthesized and tested to determine which compounds would perform the function of preventing or reducing aldehyde release from DBS enhanced polypropylene products. Adipic dihydrazide, eicosanedioic acid dihydrazide, glutaric dihydrazide, pimelic dihydrazide, sebacic dihydrazide and suberic dihydrazide were the subject compounds of the tests. Further, sulfonyl hydrazides and N-amino imides may be used as well.
The following examples serve to illustrate the subject matter of the present invention and are not to be construed as limiting the scope of the invention.
Additives were weighed and mixed with a reactor-grade polypropylene using a Welex high-intensity mixer operating at 1600 rpm. The general formulation was 1.00 kg resin, 0.500 g Irganox 1010, 1.00 g Irgafos 168, and 0.800 g calcium stearate, plus clarifier and hydrazide, when applicable. The mixed powders were then extruded on a Killion 24:1 single-screw extruder using a 400/425/450/450xc2x0 F. heating profile and pelletized. 50-mil plaques of each formulation were prepared using a 25-ton Arburg injection molder using a flat 220xc2x0 C. barrel profile.
The following method illustrates the general procedure for the manufacture of the hydrazides herein: